1. Field of the Invention
This invention relates to an infrared detector suitable for, for example, an infrared imaging device.
2. Description of the Prior Art
Photon-type (quantum-type) infrared detectors in the form of BLIPs (Background Limited Photodetector) usually include a cold shield to suppress noise. BLIPs are detectors in which noise due to fluctuations in the amount of excess electrons and positive holes generated by incident infrared rays and due to fluctuations in the recombination of both the excess electrons and positive holes is dominant, and such noise can be suppressed by reducing the amount of incident infrared rays.
The incident infrared rays can be divided into signal light radiated from an object and unwanted light other than the signal light. A shielding wall for interrupting this unwanted light is termed the cold shield. In the case of visible light, it is possible to interrupt the unwanted light by using a black shielding wall. However, in the infrared region, since an object at an ordinary temperature per se radiates infrared rays, it is necessary to devise a particular measure to suppress radiation by the shielding wall per se. One such measure is to cool the shielding wall to a low temperature. As may be seen from Planck's formula representing the radiant quantity W of a blackbody: EQU W=.sigma.T.sup.4,
where, .sigma. is the Stefan-Boltzmann constant, and T is the blackbody temperature,
the radiant quantity W can be substantially reduced by lowering the temperature. Since such a shielding wall is used by cooling to a low temperature, it is called a cold shield.
On the other hand, photon-type infrared detectors themselves are cooled in order to suppress noise due to electrons and positive holes generated by thermal excitation. For example, InSb detectors and HgCdTe detectors are cooled to the temperature of liquid nitrogen (77K) or liquid argon (80K). Since the cold shield is mounted surrounding the detector, it is usually cooled by the same means used to cool the detector.
FIG. 1 is a sectional view of a prior art infrared detector as shown on page 354 of "Infrared System Engineering", by R. D. Hudson, Jr., issued by John Wiley & Sons, 1969, wherein a container 1 having double walls is referred to hereinafter as a Dewar. Reference numeral 2 designates a Dewar window, 3 is a photon-type infrared detection element, 4 is a cold shield, 5 is a cold filter, 6 is a space for accommodating a container filled with a coolant, 7 are infrared rays to be measured, 8, 9 are unwanted or spurious infrared rays (other than the infrared rays 7 to be measured), and 12 designates a mounting substrate for the detection element 3. A container in the space 6 is filled with coolant to cool the detection element 3 and to increase the detection sensitivity. The cold filter 5 is secured to the cold shield 4, and since the cold shield is adhered to a wall portion of the container 6, the cold shield 4 and cold filter 5 are cooled together. In order to cool the detection element 3, cold shield 4, and cold filter 5 efficiently, the space enclosed by the Dewar 1 and Dewar window 2 is evacuated.
In this prior art detector the infrared rays 7 to be measured impinge on the detection element 3 after passing through the Dewar window 2, an opening 4a of the cold shield 4, and cold filter 5, and are detected. The cold shield 4 mounts the cold filter 5, and at the same time prevents unnecessary infrared rays radiated from the surrounding background at an ordinary temperature from impinging on the detection element 3 to thereby reduce the noise generated by the detection element. Although the cold shield 4 generates some emissions from its surface, since it is cooled to a low temperature the amount of unwanted infrared rays radiated from the surface of the cold shield is reduced to such an extent that it can be neglected as compared with the infrared rays 7 to be measured. The cold filter 5 transmits a desired range of wavelengths of the infrared rays 7 to be measured which impinge on the detection element 3 through the opening 4a of the cold shield 4, and suppresses the transmission of infrared rays in an undesired wavelength range to further reduce the noise. For this purpose, the cold filter 5 is formed by depositing on the surface of a substrate a coating or layer of dielectric material which selectively transmits the desired wavelength range of the infrared rays, and in which the substrate absorbs only a low degree of infrared rays in the desired wavelength range. In addition, as described above, the cold filter 5 is cooled to a low temperature in order to suppress the amount of unwanted infrared rays radiated from the cold filter per se. Since the cold filter with its deposited dielectric material has a high reflectivity for the unwanted wavelength range of the infrared rays, such rays as indicated at 8 in FIG. 1 radiated from the cold shield 4, the mounting substrate 12, and the detection element 3 are reflected by the cold filter and then impinge on the detection element. However, since the cold shield 4, mounting substrate 12, and infrared detection element 3 are cooled to a low temperature, the radiant quantity of unwanted infrared rays is negligibly small. Accordingly, detector noise due to infrared rays emitted by such elements can be neglected.
By omitting the cold filter 5, and instead forming a filter of dielectric material on the Dewar window 2, it is possible to eliminate infrared rays other than the wavelength range of the rays 7 to be measured from entering from the outside of the Dewar 1. However, with such an arrangement unwanted rays 9 radiated from the inner surface of the outer cylinder of the Dewar are reflected from the dielectric filter formed on the window 2 and impinge on the detection element 3 to cause noise. Since the outer cylinder of the Dewar is not cooled by direct contact with the coolant container space 6, this noise becomes unacceptably large.
In the prior art infrared detector mentioned above, it has been necessary to cool the detection element, the cold shield, and the cold filter in order to reduce noise.
Recently, multiple-element infrared detectors have been widely used, and the size of the cold shields used therein has become large. Thus, the heat load imposed on the cooling means has increased. As a result, a problem has arisen in that the time (cool down time) for the detection element to be cooled to a predetermined temperature has increased. Moreover, when a refrigerator such as a Stirling cycle cooler or the like is used, the power consumption of a motor and its size are increased.
Furthermore, when the size of the cold shield is increased, the manner of attaching the cold shield becomes a problem. In order to prevent the degradation of its vibration-resistance, it is necessary to mount the cold shield securely. However, it is difficult to achieve this without increasing the heat load.